1. Field of the Invention
This invention relates to improvements in a so-called f.multidot..theta. lens used in an optical scanning system.
2. Description of the Prior Art
Generally, in laser beam scanning systems, use is made of a so-called f.multidot..theta. lens in which the image height is given by f.multidot..theta. where f is the focal length of the lens and .theta. is the angle of view (the angle which an incident parallel light beam forms with the optical axis) so that the scanning velocity of the parallel light beam by rotation of a reflecting member or the like and the movement velocity of an image (a point whereat the parallel light beam is condensed) are in proportional relationship.
A laser light which effects scanning by the use of such an f.multidot..theta. lens includes much infrared light or ultraviolet light and where use is made of such light which is not directly visible to the eyes, inconveniences have been caused in the adjustment during manufacture or the setting of the apparatus when actually used. In fact, there have been required f.multidot..theta. lenses which are widely applicable by enabling laser light of two wavelengths to be used.